1) Settle a city on that small island to your west. Better, settle
two cities on islands (there's also an island to the east you could settle on if you want). They'll give you intercontinental trade routes with
all your other cities. Add that to their own trade routes, and just by building those two cities you gain over 50 commerce per turn in your empire (of course, you have to spend a little of that on new city maintenance, but not much).
2) You haven't explored all of Nappy's land, you haven't explored the rest of your continent, and you
certainly haven't put much time into naval exploration. By this point in the game, everywhere you can reach without optics you should already have explored. For reference, I went ahead and checked in the WB to see what sort of impact this would have made... suffice to say it would have made a
big difference.
It also would certainly have helped you with settling your continent, which if anything has gone too slowly - with all those gold and gems resources, you probably ought to have been able to fill in your whole continent by now, but there are several nice city spots you haven't grabbed yet. And it would be helpful for you in planning your war against Nappy right now - you don't know how many cities he has, what size they are, what sort of production they have, what resources they give access to, whether he has
other hostile neighbors...
3) As others have mentioned, you need more siege. Much more siege. Your big stack has 31 melee units, 1 scout (??), and 2 catapults. A more reasonable mix would be something like 16 melee, 16 catapults. Also, you've got a lot of espionage piled up against Nappy. If you're planning on eliminating him, that does you no good long-term, so you might as well use it for spies right now. You could manage 3-4 city revolts, which would wipe out his city defense in 3-4 cities before you attacked them... just make sure to have spies sitting stationary in those cities for 5 turns before you use them for a revolt. It
might be a little late for that in this game, but it's certainly something to keep in mind in the future.
4) More workers. Again as others have mentioned, you don't have enough.
Most of your cities are working mostly improved tiles, but...
-Dijon is working unimproved corn.
-Karakorum is working inland lake without lighthouse (2f, 2 commerce) when it could be working a grass tile you could have built a cottage on; by now that would be worth much more than 2 commerce. It's also working an unimproved grass/hill tile.
-Ning-Hsia needs a fishing boat on that fish. It's also working unimproved plains.
-Old Serai is working 2 unimproved grass tiles.
-Tolosa is working unimproved grass, unimproved plains, and a coastal tiles without a lighthouse.
-Camulodunum is working 3 unimproved grass tiles.
-Isca is working 4 unimproved grass tiles.
-Bibracte is working 4 coastal tiles despite
not working 3 grass tiles. Build cottages on those grass tiles and they'll quickly become more valuable than coastal tiles. In particular, one of those tiles
already has a cottage, but isn't being worked because the city manager AI is failing miserably.
-Verlamion is working 2 inland lakes w/o lighthouse and an unimproved grass tile. By this point, it should be working 3 grass/village or grass/town tiles instead!
-Durocorturum is working an unimproved grass tile.
-Nemetocenna is working unimproved grass/silk.
-Bagacum is working two coastal tiles, when there's two perfectly good grass tiles there you could have cottaged.
5) More thought in improvements. Your cities are small, and their food surpluses are also small. Part of this is because you don't have Civil Service. However, another part is because you don't really seem to build farms - your entire empire has a grand total of 3 farms not on wheat/corn/rice. As a rough "rule of thumb", don't build cottages until a city has a food surplus equal to it's current happiness surplus - that is, if you have a city currently with 8 happiness and 4 unhappiness, build farms instead of cottages until you've got 4 surplus food there. This isn't a scribe-in-stone law, but it's a good guideline to work by. Once the city starts to approach it's happiness cap, you can replace some of those farms with cottages if you want.
6) Tech. tree thought. You should
absolutely have gotten Civil Service by now. It's not uncommon for players to get that tech by 1500
BC actually. You got compass, but don't seem to be making any use of it - you probably could have put that off until later. In general, whenever you're going to research a tech first ask yourself if it will cost you anything to wait and research it later instead. If the answer is no, then wait and research it later.
7) Improvements. I notice that just about all your cities have Duns. Yes, it's nice to have the free Guerrilla 1 promotion... but not nice enough to build one in every city. You really only need a few guerrilla units overall - 3-5 would be plenty, to defend the stack on hills or take on top-defenders in cities. Which means you really only need a dun in 1 city. That's quite a few hammers you could have saved right there. In general, your cities are badly lacking in basic economic infrastructure. Further, I note almost no markets at all, but almost a dozen libraries. When your science slider is down in the 0-40% range
and you aren't focused on running specialists, generally markets will give you more "bang for your buck" - that is, with 100 commerce but at 20% research, a library will give you 5 more beakers, but a marketplace will give you 20 more wealth. Note the caveat I put there, though; if you're focusing on running specialists to generate science, the same does not necessarily apply. However, just looking through your cities and looking at all the cottages you built I can tell that isn't the case this game. Also, I notice you have only 3 courthouses. Ideally you'd have at least 8 by now, so you could build a forbidden palace somewhere up north following your war with Napoleon to keep your empire's costs down.
8) Civics. You're running Serfdom right now, which is almost indisputably the worst civic in the game. Build a few more workers instead, and run slavery or caste system. Generally you run slavery to help get new cities up faster, to help get ready for war quicker, or to use up surplus food and turn it into production; caste system is a very popular one for peaceful periods where you can run a lot of specialists. Similarly, I note you're in Paganism. As soon as you're done building military units (which I would stop right now, except for maybe building one last round of catapults), switch to organized religion before going back to building infrastructure. It's an extremely powerful civic and you should make full use of it whenever you can.
The big takeaways: Settle intercontinental cities. Much more exploration, more farms, more workers, more siege, more infrastructure. More thought on civics and technologies. Fewer melee units, fewer cottages. One last point - I did check this in the WB, and your tech situation isn't hopeless. Play smart and you can still win.
Boudi certainly had the traits for it.
is a major drain. By simply settling 2 cities on another island you can more than double your trade 
